The present invention relates to printheads for thermal ink jet printers, and, more particularly, to thermal ink jet printheads that have a temperature monitoring system that is used to determine when the printhead has deprimed or its ink supply has been depleted.
Thermal ink jet printing systems use thermal energy pulses generated by the heating elements in an ink jet printhead to produce momentary ink vapor bubbles on the heating elements which eject ink droplets from the printhead nozzles. One type of such a printhead has a plurality of parallel ink channels, each communicating at one end with an ink reservoir and having opposing open ends that serve as nozzles in the droplet emitting face of the printhead. A heating element, usually a resistor, is located in each of the ink channels a predetermined distance upstream from the nozzles. The heating elements are individually driven with a current pulse to momentarily vaporize the ink and form a bubble that expels a droplet of ink. The channel is then refilled by capillary action, drawing ink from a supply tank. A meniscus is formed at each nozzle under a slight negative pressure to prevent ink from weeping therefrom. Operation of a thermal ink jet printer is described, for example, in U.S. Pat. No. 4,849,774 and U.S. Pat. No. 4,571,599.
The carriage type ink jet printer typically has one or more small printheads containing the ink channels and nozzles in a nozzle face. The printheads are connected to an ink supply tank. In one configuration, the printhead and one or more ink tanks are integrally assembled and the entire configuration, sometimes referred to as a cartridge, is disposable when the ink in the ink tanks are depleted. In another configuration, the printhead is an integral part of a replaceable ink tank support and replaceable ink supply tanks are installed on the ink tank support. Generally, the ink tank support is first installed on the printer""s translatable carriage and then the ink supply tanks are installed. Each of the ink supply tanks is replaced when the ink contained therein is depleted. The replaceable ink tank support should not need to be replaced until at least ten ink supply tanks have been emptied during printing operations.
For carriage type multicolor ink jet printers of the latter type, there is a replaceable ink tank support for printing black ink and a separate replaceable ink tank support for printing non-black inks. These ink tank supports are installed on the printer""s carriage and then the respective ink tanks are installed on the appropriate ink tank support. Whether the carriage type ink jet printer uses replaceable cartridges comprising integral printheads and ink supply tanks or replaceable ink tank supports with integral printheads and separate replaceable ink tanks, both types are translated back and forth in the printing zone of the printer to print a swath of information on a recording medium, such as paper. The swath height is equal to the length of the column of nozzles in the printhead""s nozzle face. The paper is held stationary during the printing and, after the swath is printed, the paper is stepped a distance equal to the height of the printed swath or a portion thereof. This procedure is repeated until the entire page is printed or until all information has been printed, if less than a page. For an example of a typical ink cartridge, refer to U.S. Pat. No. 5,519,425 which discloses disposable ink cartridges having integral printheads and ink supply tanks, and refer to U.S. Pat. No. 5,971,531 for a replaceable ink tank support having integral printheads and separately replaceable ink supply tanks.
As is well known, thermal ink jet printheads heat up during a printing operation. If the printhead heats up too high during, for example, extended high density printing, the printhead may loose prime or become deprimed. When the printhead becomes deprimed, one or more nozzles of the printhead cease to expel ink droplets. To safe guard against excessive heating of the printhead, many prior printheads incorporate a heat sink of sufficient thermal mass and of low enough thermal resistance that the printhead temperature does not rise excessively and does not exceed the maximum operating temperature of the printhead. For one example of a printhead having a heat sink, refer to U.S. Pat. No. 4,831,390. Nevertheless, this approach does not eliminate the catastrophic printing failure mode, if printing is attempted during a printhead deprime wherein the ink in the channels retract from the nozzles and from the heating elements. In this event, the application of electrical current pulses to the heating elements without ink in contact with them causes a rapid rise in temperature of the heating elements and thus the printhead. If the printing operation is not discontinued within a relatively short time period, the printhead will be damaged or destroyed. The same problem is encountered when the ink supply to the printhead is depleted. It is especially important to stop the printing of an unattended ink jet printer when the printhead becomes deprimed or the ink supply is depleted, for continued attempted printing beyond a brief period of time will severely damage a printhead.
It is known that increase in temperature of the printhead above its normal operating temperature affects the printing quality. The printed droplet size or pixel size varies with temperature. In fact, the mass and velocity of the ejected droplet increase with printhead temperature and contribute to the increased pixel size on the paper or other recording medium.
In many existing thermal ink jet printers, various techniques are employed to maintain the printhead operating temperature within the appropriate range. For example, as disclosed in U.S. Pat. No. 4,791,435, the printing speed of the printer is slowed if the temperature of the printhead begins to rise too high. In another example, U.S. Pat. No. 5,107,276 discloses the selective energization of heating elements in the printhead not being used to print with energy pulses insufficient in magnitude to vaporize ink in order to prevent printhead temperature fluctuations during a printing operation. U.S. Pat. No. 5,036,337 discloses the varying of the energizing pulses to the heating elements to control the droplet volume. U.S. Pat. No. 4,719,472 discloses the use of a separate heater and temperature sensor to heat and monitor the temperature of the ink in the reservoir to adjust the viscosity of the ink.
Though it is known to monitor and control the operating temperature of thermal ink jet printheads to maintain print quality, there is a problem of cost effectively identifying a deprime of the printhead or an empty ink supply container. This is especially a problem when the printer is being operated at a remote location or for an unattended operation, where a user cannot see that the printhead has stopped ejecting ink droplets. The known methods of detecting the presence or absence of ejected ink droplets from printheads during a printing operation are generally complex and expensive, while the aim of this invention is to determine such event in a simple cost effective manner.
It is an object of the present invention to provide an ink jet printhead having means to detect absence of droplet ejection during a printing operation by utilizing information from the printhead temperature monitoring system.
In one aspect of the present invention, there is provided a thermal ink jet printhead for an ink jet printer having means to determine when the printhead has stopped ejecting ink droplets during a printing operation, comprising: a structure having an ink supplying reservoir with an ink inlet thereto, a plurality of droplet ejecting nozzles, and a plurality of capillarily filled ink flow directing channels that interconnect the reservoir to each of the nozzles; a plurality of selectively addressable heating elements, one heating element being located in each channel a predetermined distance upstream from the nozzles, the heating elements producing momentary ink vapor bubbles when energized to eject ink droplets; a temperature sensor for sensing the temperature of said structure; and a control circuit for selectively applying electrical signals to the heating elements for energization thereof in response to image data signals received thereby, the control circuit including a memory for the storage of a predetermined temperature indicative of the maximum allowable operating temperature of the structure, means for comparing the sensed temperature of said structure with said predetermined temperature stored in said memory and generating a signal whenever said sensed temperature is greater than the predetermined stored in said memory, whereby a signal generated by said means for comparing and generating in said control circuit is indicative of a stoppage of droplet ejection by said structure during a printing operation.